1. Field of the Invention
The present invention relates to a printing apparatus which carries out printing on a printing medium by discharging a liquid from a nozzle.
2. Description of the Related Art
In an ink-jet printer described in Japanese Patent Application Laid-Open No. 2009-06550 as a printing apparatus which carries out printing by discharging a liquid from a liquid discharge head, an ink-jet head includes: a flow passage unit in which a plurality of nozzles and an ink flow passage including a plurality of pressure chambers communicating with the nozzles are formed; and a piezoelectric actuator arranged on the upper surface of the flow passage unit to apply a pressure to an ink inside the pressure chambers. The piezoelectric actuator has three piezoelectric layers which are stacked on the upper surface of the flow passage unit, and common electrodes and individual electrodes which are formed respectively on the lower surface and upper surface of the uppermost piezoelectric layer so as to sandwich the uppermost piezoelectric layer on the portions facing the plurality of pressure chambers. Further, the ink-jet head is a so-called line head which extends across a full length of a recording paper in its width direction and is fixed to the ink-jet printer.
Then, when a drive pulse is applied to an individual electrode to generate a potential difference between the individual electrode and a common electrode which is maintained at the ground potential, in the same direction as the direction of polarization of the piezoelectric layer, a downward electric field is generated in the portion of the piezoelectric layer sandwiched by those electrodes. This electric field causes the uppermost piezoelectric layer to contract in a horizontal direction, thereby deforming the three piezoelectric layers at the portions (a piezoelectric element) facing a pressure chamber, as a whole, to project toward the pressure chamber side. Thus, a pressure is applied to the ink inside the pressure chamber.
Here, in the ink-jet head described in Japanese Patent Application Laid-Open No. 2009-06550, as a total number of times of driving a piezoelectric element increases, the polarization of the piezoelectric layer weakens, etc., thereby degrading the drive performance of the piezoelectric element. Therefore, when a drive pulse of a certain height (drive potential) is applied to drive a piezoelectric element, the maximum drivable number of times is predetermined for the piezoelectric element. Thus, if a particular piezoelectric element has a high usage frequency, and the total number of times of driving the piezoelectric element with the high usage frequency will reach the maximum drivable number of times described above, the life of the piezoelectric element ends at an early stage. In this case, even if the other piezoelectric elements are still drivable, the ink-jet head will have to come to the end of its life.
At this time, if the ink-jet head is a so-called serial head which discharges ink while moving in a scanning direction and therefore differs from that described in Japanese Patent Application Laid-Open No. 2009-06550, it is possible to prolong the ink-jet head life by, for example, discharging the ink from another nozzle to the position to which the nozzle corresponding to the expired piezoelectric element should have discharged the ink on the recording paper. However, it is difficult for a line head as described in Japanese Patent Application Laid-Open No. 2009-06550 to carry out such kind of process.